Internal combustion engines can be powered with a variety of fuels such as gasoline, diesel fuel, natural gas, liquid petroleum gas, or fuel mixtures such as gasoline/methanol or gasoline/ethanol. Dual fuel engines have also been invented which use diesel/natural gas or diesel/propane fuels, for example. Internal combustion engines produce large quantities of exhaust gases consisting primarily of carbon dioxide, water, nitrogen, oxygen, partially combusted and uncombusted hydrocarbons, carbon monoxide and oxides of nitrogen. It is well known in the art to employ an exhaust gas converter containing an oxidation catalyst to treat exhaust gases in order to reduce the concentrations of pollutants such as uncombusted hydrocarbons, and noxious by-products. However, in order to efficiently oxidize pollutants in exhaust gases, the catalyst must operate at high temperatures. Conventional converters therefore exhibit poor conversion efficiency at low engine loads due to low exhaust temperatures. This leads to increased exhaust emissions during low load operation, especially for the non-reactive hydrocarbons, specifically, methane. When a diesel engine is idling and the exhaust gas temperature falls below 300.degree. C., emission reduction in the catalytic converter is lessened because the temperature of the exhaust gases is cooler than the light off/ignition temperature of the catalyst. This is particularly a problem when the engine is a dual fuel engine powered by a diesel fuel/methane mixture. To overcome this problem, reversing flow catalytic converters have been invented.
A reversing flow catalytic converter works on a principle of periodically redirecting engine exhaust through a catalyst in alternate directions. The duration of flow in each direction is determined by engine operating conditions. The goal is to obtain an ideal temperature profile throughout the catalytic material in the catalytic converter. For example, in a PCT patent application PCT/US97/19928, which was published on May 14, 1998. Matros et al. discloses a method and a system in which exhaust gases in contact with a gas permeable solid material containing an adsorbent and a catalyst capable of converting noxious components in the exhaust gases into innocuous substances. The flow of gases through the gas permeable solid material is reversed in a series of continuing cycles to bring, or to maintain, the catalyst in a temperature range suitable for oxidizing the noxious components. Below that temperature range the noxious components are adsorbed by the adsorbent. One embodiment described in this application comprises four valves working co-operatively to achieve the full reversing function. A disadvantage of this embodiment is that the structure is bulky because of the required plumbing and valving.
In a second embodiment, reversing the flow of the exhaust gases through gas permeable solid material is achieved by axially rotating the solid material while the gas flow direction through inlet and outlet ports remains unchanged. Rotating of the solid material moves the material from a first heat exchange zone to a second heat exchange zone in a repetitive cycle. The gas permeable solid material has a plurality of parallel axial channels and the exhaust gases are passed through one section of the channels in a first direction and then are passed through another section of the channels in the opposite direction. The catalyst is preferably applied to the surface of substantially all channels in the rotating element adjacent to an inlet and an outlet for receiving and discharging the exhaust gases. The adsorbent is applied to the surface of substantially all channels adjacent to a space where the exhaust gases change direction of movement.
In a third embodiment, the rotating element is cylindrical and has a hollow central interior. A plurality of radial channels communicate with the hollow central interior. Those channels provide gas passages from a lateral side of the rotating element adjacent to an inlet port to the hollow central interior, and from the hollow central interior to the other side of the rotating element adjacent to an outlet port. The catalyst is applied to the outer portions of the cylindrical element. An adsorbent is applied to the inner portion adjacent to the hollow central interior. Both the second and third embodiments require the rotation of substrates to which the catalysts are applied, rather than changing the direction of the gas flow.
A disadvantage of each of the structures described by Matros et al is that they are not compact. For example, in the second embodiment a closed compartment 21 is required at one end of the first and second heat exchange zones to provide a stationary passageway for gas flow from the moving channels in the first heat exchange zone to the moving channels in the second heat exchange zone (FIG. 6). Furthermore, the reliability of performance is compromised because of the rotating structure.
Instead of using four co-ordinated valves to control the reversal of gas flow, or a rotating substrate structure, a four-way valve provides a more reliable structure for reversing flow converters. In a paper entitled "Novel Catalytic Converter for Natural Gas Powered Diesel Engines to Meet Stringent Exhaust Emission Regulations" which was published in the Proceedings of NGVs Becoming a Global Reality, International Conference and Exhibition for Natural Gas Vehicles, May 26-28, 1998, Cologne, Germany. Zheng et al describe a catalytic converter which has a four-way valve to switch the direction of a reversing gas flow. The four-way valve is a universal valve, structurally independent of the converter and directs flow radially. Therefore, the plumbing required for the converter makes the system quite bulky.
Another converter structure is described by Houdry et al. in U.S. Pat. No. 3,189,417 which issued on Jun. 15, 1965, and is entitled "Apparatus for Improving the Purification of Exhaust Gases from an Internal Combustion Engine". This patent discloses a reversing flow converter which has a bed of oxidation catalyst pellets confined between two layers of heat exchange material. A four-way valve is incorporated in the converter. When the valve is rotated 90.degree., the direction of the gas flow is changed from passing downwardly through the catalyst bed and heat exchange material to passing upwardly through the bed in an opposite direction. This arrangement of a bed of oxidation catalyst pellets separate from the heat exchange material is not efficient and conversion performance is poor. Also, because of the structure of flow passages and the manner in which the valve is incorporated in the structure, the structure is not compact.
As a further example, a four way valve construction is taught in U.S. Pat. No. 4,139,355 which issued to Turner et al on Feb. 13, 1979 and is entitled "Four Way Valve For Reversible Cycle Refrigeration System". This patent discloses a four way valve assembly in which a rotary valve accomplishes switching between heating and cooling modes. The rotary valve is mounted in a cavity in a housing and is designed to be rotated by a unidirectional electric motor. The rotary valve comprises a rotating plate having a pair of recesses in it. Each recess provides fluid communication between a pair of ports in a base plate. In order to balance the high pressure acting on one side of the rotating plate, a high pressure bypass port is provided to balance the pressure in the cavity. A cam and switch arrangement provides the necessary control to stop and start the electric motor. This rotary valve, however is not suitable for use in a reversing flow catalytic converter due to its structure. In particular, all four ports are located in the base plate.
The concept of the reversing flow catalytic converter has been demonstrated to be sound and to contribute to reduced exhaust emission levels. However, modern vehicle design demands compact, efficient and mechanically reliable components. Each of the prior art catalytic converter structures described above fail to meet at least one of these criteria.